This invention relates to apparatus for injecting molten metals into a mold cavity, said apparatus being commonly referred to as a "cold chamber," and more particularly to an improved "shot tip" which serves as the plunger or ram in the injection apparatus.
It is well-known that molten metals such as aluminum, zinc, magnesium and other metals and alloys of same can be injected into a mold cavity by means of a device known as a cold chamber. This well-known device comprises a cylindrical sleeve having a through bore which is adapted to receive a plunger or "shot tip." The sleeve is provided with a radial opening called a "well" through which the molten metal is introduced to the interior bore of the sleeve or chamber. After the metal has been introduced and accumulated in sufficient quantity, power means are activated to drive the shot tip forward, injecting plunger-fashion the molten material into the mold cavity.
Cold chambers or shot sleeves and particularly shot tips operate in an extremely hostile environment as far as thermal strain and wear is concerned. Accordingly, the devices typically exhibit a short life span.
One of the principal problems giving rise to the short life of the prior art shot tip is the extreme heat experienced by the face of the shot tip, that is, that portion of the shot tip which comes into contact with the injected molten metal. Cooling is attempted by hollowing out the shot tip and creating a coolant water conduit axially into and out of the hollow area. However, it is believed that the prior art arrangement which involves pumping water through a central tube and exhausting around the outside of the tube is inefficient because the coolant water experiences a dramatic temperature rise and vaporization as it emerges from the tube and impacts the extremely hot front wall or face of the shot tip. When vaporization occurs, the pressure within the cooling chamber increases to the point where it is greater than the line pressure of the water and, at least for an instant, the flow of coolant is interrupted or slowed. Moreover, inefficient flow within the hollow chamber results in much of the water passing through without carrying heat away. High heat causes thermal expansion and inordinate wear on the leading edge of the piston.
In applicant's copending U.S. application Ser. No. 834,590, filed Feb. 28, 1986, of which this application is a continuation-in-part, an improved shot tip design is disclosed and claimed comprising a head which has formed therein an internal chamber to receive coolant through a passage defined by the radial space between the outer surface of an outlet tube and the inner diameter of a bore in the shot tip head. Means are provided to cause the coolant to follow a smooth flow path around the chamber and into the tube end in such a fashion to drastically improve the rate at which heat is carried away from the face or leading edge of the piston. The shot tip is formed with an undercut or shoulder defining a narrow forward facing annular step surface a substantial distance longitudinally back from the nose of the shot tip such that the majority of the molten metal contacts the nose surface. This, in effect, makes the large area and large volume nose portion a heat sink which results in the cylinder-contacting peripheral surface adjacent the shoulder being cooler and less susceptible to wear producing thermal expansion. The shot tip of this copending application exhibits a number of major changes as compared to industry standards.
Firstly, the coolant or water is pumped into the piston in reverse fashion. Instead of pumping water into the piston by a copper tube, the flow is altered and the water is exited via the copper tube. This means that the water then enters the piston via a hole in the plunger rod or actuating rod. Since the copper tube is concentric with the hole in the rod, the water enters the piston through the area between the wall of the inner diameter and the copper tubing.
Secondly, the copper tube protrudes into the piston from the plunger rod. Because the water path is reversed as explained above, a baffle can be attached on the end of the tubing to control not only the flow path of the water but also its velocity through the piston by varying the size of the baffle. By increasing the size of the baffle, the area through which the water passes is decreased, thereby increasing the water's velocity. It has already been determined that to achieve optimum cooling from the circulating water, it should travel at a rate close to 10 feet/per second. It must also contact as much surface area as possible of the object intended to be cooled. To help direct the water out of the piston, a deflector is provided which is mounted on the end of the baffle. The deflector directs the hot water or steam back through the inner diameter of the baffle and into the copper tube which is attached to the baffle.
Thirdly, the flow path of the water is such that it will reach the hottest part of the piston directly in front of the exhaust hole leading into the copper tubing. If the water is heated to its flash point, the steam that is formed will not interfere with the incoming water supply. In fact, when steam is passed in the copper exhaust tube, the incoming water which surrounds the copper tube will condense the steam and form a vacuum. This vacuum creates a syphon effect which pulls the water along to replace the vacuum. This system ensures that any increase in volume produced by the creation of steam will not form a back pressure and prohibit or slow the flow of water through the piston.
Fourthly, even though the rate at which the piston is cooled has been increased, it is still impossible to eliminate high heat (1250 degrees Fahrenheit) across the entire face of the piston. Therefore, an undercut is incorporated bringing the leading edge of the cylinder contacting portion of the piston back along the diameter. For example, the shot tip may be undercut 0.625 inches and as much as 1.0 inches. The object of this undercutting is to bring the water cavity forward in the piston, increasing the surface of the tip nose which contacts the molten metal, causing this nose to act as a heat sink and reduce radial expansion in the part of the tip which slides against the cylinder wall. The advantage of this design is that expansion across the diameter at the point of the leading edge which must seal against the sleeve can be better controlled. This expansion control greatly improves tip life since excessive wear caused by the expansion of the piston against the shot sleeve wall is the primary cause of tip failure.
The present invention may be utilized alone or to even greater advantage in combination with the invention of Ser. No. 834,590 to address the problem of reducing tip wear and tip failure due to a tendency of the tip to hang up or drag along the walls of the sleeve under certain circumstances.
The drag problem occurs primarily because the sleeve tends to warp into a curved configuration as a result of the heat of the molten metal being primarily transferred to the bottom of the sleeve; i.e., when hot metal enters the sleeve, the bottom of the sleeve expands at a much greater rate than the top of the sleeve, forcing the sleeve to warp. This warpage is greatest immediately beneath the pour hole in the sleeve because the greatest amount of heat is transferred through the sleeve at that point and because the pour hole above this hot spot offers less resistance to the expanding bottom so that a defined bend occurs at the pour hole and this bend often causes the shot tip to hang up or drag in the sleeve as it traverses the area of the pour hole.